1. Field of the Invention
The present invention generally relates to AC to DC power converters and, more particularly, to design strategy for increasing power density of such power converters through reduction of electromagnetic interference (EMI) filter size in such power converters.
2. Description of the Prior Art
The great preponderance of electrical and electronic devices currently in use contain circuitry which is designed to operate using direct current (DC) power while, for reasons of distribution efficiency, most power is ultimately delivered to such devices as alternating current (AC) power. Therefore, many electrical and electronic devices include power converters to develop DC power at a desired voltage from distributed AC power. Such power converters are currently required to include circuitry which provides power factor correction (PFC) and prevents the device from causing electromagnetic interference noise from propagating from the device or power converter to the AC power distribution system in order to preserve the quality of the AC power and to prevent propagation of noise between devices connected to the AC power distribution system. The EMI filter also serves to protect powered devices from EMI noise which may be present on the AC power distribution network.
In high power applications, paralleling of devices (e.g. active and passive components) in power converters is virtually inevitable in order to handle and deliver large currents which are often required. However, it is often the practice to provide power converters in parallel (often referred to as a modular approach to overall power converter design) rather than connecting the individual, respective components thereof in parallel in a single power converter. A modular approach is considered to be more effective than connecting individual components in parallel since it allows control of current sharing between the parallel connected power converters and can be effective to reduce ripple in the DC output voltage which, in turn, can reduce the size and cost of output filter arrangements. Accordingly, a modular approach is widely used in all types of distributed power systems such as data server and telecommunications applications.
In such modular power converters, an EMI filter and PFC circuit is built into the front-end power converter module. Accordingly, the size and cost of EMI filters and PFC circuits is significant, particularly in the aggregate over large numbers of power converters. However, when plural power converters are used in parallel to supply adequate power for a single device or a plurality of devices proximate to a single location or to each other (e.g. as in a vehicle), the PFC circuits can be operated in multiple interleaved channels equally separated in time (with the phase separation being 360°/m where m is the number of channels, generally referred to hereinafter as symmetrical); allowing a single EMI filter to be connected in common to all channels. By doing so, the EMI filter size can, in some cases, be reduced due to ripple cancellation effects in the converter or power factor correction (PFC) circuit which reduce the magnitude of differential mode (DM) EMI noise. It is also possible to connect PFC circuits in parallel such that the number of PFC circuits required is different from the number of DC/DC power converters that may be required in a given application. Another benefit of interleaved channels is that the ripple frequency is increased in frequency by a factor equal to the number of channels employed. In some cases, this increase in ripple frequency increases the “corner frequency” (e.g. the nominal frequency of the low pass attenuation) of the EMI filter, allowing smaller components to be used in the EMI filter although there is a trade-off between the size and cost of the EMI filter and the size, cost and number of the PFC channels that may be used. Thus, the size and cost reduction in EMI filters achieved through operation in symmetrical interleaved channels is quite limited and size and cost remains significant at the present state of the art.